1. Field of the Invention
The present invention relates to a safety system to detect and annunciate when a transit vehicle, such as a train operating on rails and controlled by an automatic train control system, experiences a loss of occupancy detection. When a loss of occupancy detection occurs the train control system believes there are no trains in that section, called block, of track and would allow an oncoming train to enter the already occupied block causing an unsafe condition or worse—a collision.
2. Discussion of Background
With the advent of high-speed close headway rapid transit systems operating on rails such as the Bay Area's Rapid Transit system, BART, and Municipal Railway, MUNI, in San Francisco Calif. it is imperative that the systems controlling these trains know exactly where the trains are at any point in time.
It has been, and still is, an ongoing problem detecting with absolute certainty and reliability where a train is on any section of track without implementing very sophisticated, and often financially restrictive, presence detection equipment coupled with redundant backup systems that unfortunately impact passenger service. This is because of an operating conflict between safety and a transit system's desire to move the maximum number of passengers from point A to point B in the shortest period of time with a high degree of operating reliability. This is exhibited in the design of roads and highways that use signs and lights to display the legal speed limits at which it is deemed safe to travel and still reach your destination in a timely manner. Roadway system designers know that you can move twice the number of people in a car at 120 miles per hour as you can at 60 miles per hour—but not without increasing the risk of an accident to a level of certainty deemed unacceptable for passenger safety.
The vast majority of rapid transit systems today, such as BART or MUNI, have a minimum of two cars in a train, sometimes called consist, with cars at the beginning and end of the train having identical Automatic Train Operation (ATO) electronics. This is because rapid transit systems use parallel fixed track structures with each track having only one normal direction of travel. As such, train direction is reversed by the train operator relocating to the opposite end of the train and central control physically switching, via track switches, the train onto the parallel track to run in the reverse direction.
The majority of existing rapid transit systems control the speed and location of trains by using duel mode track signaling and occupancy detecting systems built into the running rail tracks and controlled by wayside Automatic Train Control (ATC) systems. These systems transmit predetermined speed commands to the trains, as a function of track occupancy, grade, and position, to the front of the train in essence pulling it along. Train detection is accomplished by removing these speed commands, using the train's wheels to short out the signals, normally received by track receivers that are physically located behind the train. These receivers in turn communicate with trailing track transmitters that transmit speed commands to following trains. It is important to note that although Automatic Train Operation (ATO) systems are present in both the leading and trailing cars of a train, and the trailing car's ATO system deactivated, the trailing block immediately behind the train should never be transmitting a non-zero speed command—this is paramount to this invention. The inherent safety-operability paradox in this type of control system is the minimum amount of signal the train needs to proceed versus the amount of signal the trailing receiver sees after shunting by the trains wheels. If the signal received by the leading car's on-board ATO is too low the train won't proceed and if the signal received by the trailing block's wayside receiver is to high the ATC system assumes no train is in the block.
The most common problems associated with occupancy detection systems that use trains wheels to short out the speed command signal are poor electrical contact caused by: rusty rails, contact between the train wheel-rail interface, and short signaling block lengths—all of which are dependent on running rail resistance. Of additional concern is the reduction, or sometimes total loss, of speed command signals due to leakage of track signals, often caused by rain, into the earth. This results in what is called false occupancy, or FO's, and will stop a train's movement until cleared.
As these detection problems became known throughout the transit industry attempts were made to back-up the primary detection system by alternate means. One such attempt, exemplified by BART, was to install a separate computer system that does not allow a block that was previously occupied to be cleared until the next sequential block in a train's path is detected. This system is still in use at BART today and is called the Sequential Occupancy Release System, or SORS.
Although this system does increase safety its penalty on system throughput is severe especially whenever a false occupancy occurs. This is because whenever a false occupancy occurs there is no real train on the track and therefore the block cannot be released by occupying the next sequential block because there is no train to enter the block. The problem can only be resolved by human intervention and therefore is susceptible to error as, after repetitive false occupancies, operators become conditioned by the event and manually clear the block—when in fact there is a train is in the block.
In order to resolve these concerns in a timely and cost-effective manner a solution must be found that: is compatible with the existing system, does not degrade passenger service, increases passenger throughput and above all passenger safety—all of which this present invention, as follows, uniquely satisfies.